Hydrates are a porous solid which is formed primarily of water with a mixture of gases. It is effectively similar to crushed ice which is stuck together. There is a tendency for hydrates to form in pipelines departing from a subsea gas well, especially during well flow startup.
The temperature of the seawater at depths will often approach 34° F., with the temperature in a non-blowing pipe being the same. When a subsea valve is opened, the gas expansion can cause substantial additional cooling. In these cold and high pressure conditions, hydrates of the gas and water quickly form.
Frequently when the hydrate forms, it forms a blockage. The blockage will typically be somewhat porous. At that time, a high pressure will exist on the upstream side of the blockage and a lower pressure will exist on the downstream side. This means that additional gas will move thru the hydrate and expand and therefore cool as it does. This means that not only can the expansion of this gas keep the formed hydrate cool, but can literally continue to grow additional hydrate blockage.
Attempts have been made to enter the accessible end of the pipeline with a somewhat flexible string of steel coiled tubing to get to the blockage and wash it out. This is an expensive operation due to the cost of the equipment and the time delay in arranging for and deploying the equipment. As the blockage most often occurs at the opposite end from the accessible end, the blockage can be 5 or more miles away. Removal by the use of coiled tubing is further complicated if there are bends in the pipeline, making it more difficult to impossible.
Another method of prevention of the formation of hydrates is to place expensive, environmentally harmful chemicals at the likely location of the formation of the hydrates. Chemicals such as methanol will reduce the temperature of the liquid/gas combination required for the formation of the hydrates, hopefully below the temperature which occurs during the well startup operations. In addition to the chemicals being expensive, the delivery of the chemicals to the remote location of the likely hydrate formation requires expensive capital equipment, and the cost of remediation of the chemicals is costly in some cases.
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